How do I increase the number of circle vertices? (Blender 2.8)
In Edit Mode, select all the vertices (A), right click and choose "Subdivide" (W if you use right click for select), then open the boxed dialog at the bottom left of the Viewport (also "Edit > Adjust last operation") and increase Smoothness to 1.0. You can select the multiplier (number of subdivisions) too.





Alternatively, at the moment of creation of the circle (top of Viewport, Add > Mesh > Circle) you had the option of selecting the number of vertices: this is done again by opening the boxed dialog at the bottom left of the Viewport and changing the number of vertices. But it can't be done once you have done any other operation.